CA1163150A

CA1163150A - Method of forming a secondary emissive coating on a dynode

Info

Publication number: CA1163150A
Application number: CA000349111A
Authority: CA
Inventors: Alan G. Knapp
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-04-02
Filing date: 1980-04-02
Publication date: 1984-03-06
Also published as: SE8002445L; ES8200794A1; IT8021048A0; AU5695880A; ES490108A0; US4395437A; FR2453494A1; IT1130372B; GB2048561B; DE3011381C2; FR2453494B1; DE3011381A1; GB2048561A

Abstract

PHB. 32,649. 13 ABSTRACT:

A method of forming an emissive coating on a dynode substrate. The method comprises the steps of vapour depositing afor example by evaporating or sputter-ing) a composite coating consisting of magnesium and aluminium onto the dynode substrate. A 50 to 500 .ANG. thick layer of aluminium is vapour deposited over the composite coating and the aluminium layer is oxidized. The coated dynode is then activated by heating it in an oxygen atmosphere at a pressure of at least 5 x 10-6 Torr oxygen at a temperature between 270 and 400°C. The resulting secondary emissive coating contains from 1.5 to 90% by weight of magnesium. The coated dynodes are used in channel electron multipliers which are suitable for use in electron display tubes such as image intensifiers or colour television display tubes.

Description

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3.3.80 l PHB.32649 "Method of forming a secondary emissive coating on a dynode"

The present invention relates to a method of forming a secondary emissive coating on a dynode, to a dynode coated by such a method, to an electron-multiplier and to a channel electron multiplier comprising a stack of such dynodes, and to an electron display tube, for example a cathode-ray tube or an image intensifier, including such a channel electron multiplier.
United Kingdom Patent Specifications1,401,969, 1,402,549 and 1,434,053 describe different types of channel electron multipliers, which each consist essentially of a stack of perforate metal electrically conductive layers each having a regular array of apertures with the apertures of each of said layers aligned with those of the other conductive layers in the stack so as to define the channels, and separating means disposed between each pair of adjacent conductive layers, which separating means do no-t obstruct the channels. When the conductive layer material is not sufficiently secondary emissive for a particular application, the secondary ernissive properties of the conductive layers can be enhanced by providing a coating of a more ernissive material at least on the exposed surfaces of the conductive layers inside the channels.
This may be done on all the conductive layers, but it may be preferably to apply the emissive~coating only to the -first few conductive layers located on -the input side of the channel electron multiplier.
United Kingdom Specification 1,523,730 describes dynodes suitable for use in channel electron multipliers, the dynocles consisting of substrates bearing secondary emissive coatings of cermets of specified compositions, each containing an alkali metal ~luoride.
~Iowever, the secondary emissive coefficients o~ these cermets are not appreciably more than 4.
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PHB 32649 2 4.3.80 A~ article "Growth of MgO films with high se-condary emission on Al-Mg alloys" by B. Goldstein and J. Dresner in Surface Science, Vol. 71 No. 1 (197~), pages 15-26, disclosed the formation of secondary emissive layers by the oxidation and activation of high purity sheet Al-Mg alloys having Mg-contents of from 0.1 to 3% by weight. The magnesium concentration in the surface oxide layers was increased by heating oxidized alloy sheet at temperatures of the order of 4500C, and values of the secondary emission coefficient (~ ) of from 10 to 15 were obtained by this method. These alloys are not suitable for making dynodes of channel electron multipliers since an etching technique is not a-t present available which is suitable for etching the geometries desired for dynodes in these magnesium alloys.
During the investigations which led to -the pre-sent invention, it was found that secondary emissive coa-tings formed by evaporating aluminiu~-magnesium alloys containing from 0.1 to 3~ by weight of magnesium could not be activated readily.
It is an object of the invention to provide a secondary emissive coating having higher values of the secondary emission coefficient (~ ) than the cermet materi-al known from the United Kingdom Specification 1,5239730.
It is another object of the invention to provide a method of forming a secondary emissive coating which can be acti-; vated readily.
The present invention provides a method of for-ming a secondary emissive coating on a dynode, the method comprising the steps of vapour depositing a composite coating at least 200 ~ thick consisting of magnesium and aluminium onto the dynode, vapour depositing from 50 to 500 ~ of aluminium over the composite coatingt oxidizing the exposed aluminium layer, and activating the coa-ted dynode by heating it in an oxygen atmosphere at a pressure of at least 5 x 10 6 Torr, preferably from 5 x 10 to 4 x 10 Torr at a temperature between 270 and 400C, wherein the secondary emissive coating contains from 1.5 ~ ~315(~

PHB 32649 3 4.3.80 to 90% by weight of magnesium.
The composite coating may consist of a magnesium layer disposed on a subjacent aluminium layer which ab-uts the dynode. The composite coating may be formed by vapour depositing a layer of aluminium onto the dynode, and vapour depositing magnesium and aluminium simultaneously onto the aluminium layer abutting the dynode.
The purpose of depositing an aluminium layer over the magnesium-containing layer is to provide a barrier be--tween the magnesium in this magnesium-containing layer air, since the coated dynode will be exposed to air when it is removed from -the vapour deposition atmosphere and before the coating is activated. If the surface skin of the coa-ting contained a significant quantity of magnesium, the magnesium-containing layer is difficult to oxidise - this is probably due to the formation of a magnesium hydroxide layer which must be decomposed before a MgO layer can be produced. When the magnesium-containing layer is covered by an aluminium layer, at least the outer thickness of this ~
20 aluminium layer is c~onverted by oxidiation into aluminium oxide, and magnesium diffuses into this aluminium oxide layer during the activation step, and is oxidised when it reaches the~surface of the coating.
Dynodes coated by a method according to the in-25 vention~ are used to make channel electron multipliers whichcomprise a stack of c~ted d~nodes separated from each other by separating means disposed between each pair of adjacent dynodes, each dynode having a regular array of aperture$, wherein the apertures of the respec-tive dynodes are aligned 30 so as to form the channels, wherein t~e supporting means do not obstruct the channelsl wherein the separating means are electrically insulating or have a higher electrical resistivity than that of the dynodes. The dynodes consist of single sheets or of two mating sheets which are in elec-35 trical contact with each other~ The dynode material may becoated so as to improve adhes~ion with inter-dynode insula-ting material, for example glass, and to act as a diffusion ;~ barrier to impurities, for example, sulphur, so that the ::: ~

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PHB 326~9 4 4.3.80 impurities do not polson the emissive coating.
Preferably, the dynodes consist of mild steel, since well-established etching techniques can be used to produce desired geometries of the dynodes in sheet mild 5 steel.
The magnesium and aluminium may be vapour de-posited by evaporation, since these metals are easily eva-porated. The magnesium-aluminium layer may be, for example Prom 1000 to 2000 ~ thick. When the dynode consists at least substantially of mild steel, an aluminium layer from 100 to 1000 ~ thick may be disposed between the mild steel and a magnesium-comprising layer so as to reduce the rate of diffusion of magnesium into the mild steel.
An emissive coating consisting solely of magne-sium is more difficult to activate, needing an activationtemperature of 300C, than an emissive coating produced by a method according to the invention. It is possible to activate the coatings formed by the method according to the present invention by heating in an oxygen atmosphere at a pressure of from 5 to 400 x 10 Torr for 3 hours at 270C and this is,appropriate when dynodes are activated inside an electron-display tube having an envelope with a pressure-bonded seal comprising a lead sealing member. When using higher pressure oxygen atmospheres and also when using activation temperatures above 300C it is desirable to actlvate the dynodes outside the tube so as to avoid gross oxidation of other tube components.
It is possible to activate the dynodes outside an ~electron tube - this avoids heating other components of the tube in an oxidising atmosphere~ but- there is then the risk of contaminating the activated s~condary emissive surfaces.
- When the metals used to form the emissive coating : : .
are evaporated onto dynodes, the dynodes may be at room temperature and the pressure in the evaporation chamber is preferably from 1 to 3 x 10 5 Torr~ the atmosphere in the evaporation chamber then consisting mainly of water vap~ur. Although carbon con-tamination of the dynode surface , :

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PHB 32649 5 4.3.80 has the effect of degrading the secondary emission coeffi-cient of the emissive coa-ting if this contamination is not removed, the effect of surface contamination of the dynode with carbon on the activated dynode is reduced to a low level since this contamination is reduced during the acti vation process in 15 minutes from 30~o of a monolayer to less than 5~o of a monolayer.
It was found that the maximum in the ~ -voltage curve for emissive coa-tings formed by a method according to the invention is at higher voltages (about 600 volts) than was the case for gold cryolite cermet layers of United Kingdom Patent Specification 1,523,730. This feature may be advantageous with respect to space charge problems on account of the higher currents flowing through the dy-nodes. The emissive coatings formed by the method accor -ding to the invention are more stable to electron oombard-ment than are the cermets formed with alkali metal fluori-des, and there is no :risk of fluorine contamination of electron tube components when using aluminium-magnesium emissive coatings.
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: Two embodiments of the present invention will:now : be:described with reference to the ~xamples and to the drawings in which:
~igure 1 is a schematic side-sectional elevation of an apparatus used to evaporate a secondary-emissive :
coating on a dynode substrate by a method according to the nvention, Figure 2 is a side-sectional elevation of part of a channel~electron multiplier produced from dynodes.coated :~ 30 by a method according to the invention, : Figure 3 is a side-sectional elevation of part ~: of another channel electron multiplier produced from dynodes coated by a method according to the invention, and Figure 4 is a diagrammatic longitudinal section of a channel plate cathode-ray tube including a channel electron multiplier as described with reference to Figure

2 or to Figure 3.
- Referring to Figure 1, a dynode substrate 1 is i 1~31~

PHB 32649 4-3.80 mounted on a rotatable work-holder 2 (the means used for rotating the work holder 2 are not shown for the sake of clarity) inside an evaporation vessel 3 mounted on a pump table 4. The evaporation vessel 3 contains a magnesium source consisting of a molybdenum boat 5 having a perfo-rated cover 6, the boat 5 containing a charge of magnesium, and an aluminium source which is a tungsten helix 7 which supports pieces of aluminium wire (not shown). The dynode substrate 1 is disposed at a dlstance d (20 cms) above the molybdenum boat 5 and the tungsten helix 7 9 and the dis~
tance s between the centres of the molybdenum boat 4 and the tungsten helix 7 is 2 cms. The aluminium and magnesium sources are heated by passing current from respective power supplies 8 and 9 through the helix 7 and through the boat 5, respectively. It appears, that provided that the ratio d : s is at leat 10:1, the composition of a magnesium-aluminium alloy deposited on the dynode substrate 1 by simultaneously evaporating magnesium and aluminium is homo-geneous over the area of the dynode substrate 1.
20 EXAMPLE 1.
A mild steel plate 1 which had been plated with ljum of nickel was placed on the work-holder 2 in the apparatus described with reference to Figure 1. The work-holder 2 was rotated at 30 r.p.m. Pressure in the apparatus 25 was reduced to 2 x 10 5 Torr, and -the aluminium source 7 was energised and formed at 100 ~ thick aluminium layer 10 on the mild steel plates in 2 minutes. Evaporation from the aluminium source 7 was continued and the magnesium source 6 was energised, a 500 ~ thick layer 11 consisting of 40% by weight aluminium and 60~ by weight magnesium was deposited in 3 minutes. Magnesium deposition was then stopped, and a 75 ~ thick layer 12 or pure aluminium was deposited over the magnesium-aluminium layer 11 . The coating plate 1 was then lef-t in air at atmospheric pres-sure at 20C for 60 hours so as to conver-t the surface aluminium layer 12 into aluminium o~ide. The coated plate was then activated by heating at 4 hours in a partial pressure~of 4 x 10 5 Torr oxygen. The secondary emission , ~ ~33l~

PHB 32649 7 4.3.80 coefficient ('S ) of the activated coating was 5.6 at 500 eV.
EXAMPLE ~.
Mild steel plates which had been plated with l/um nickel were placed ln the apparatus described with refe rence to Figure 1. Pressure in the apparatus was reduced to 2 x 10 5 Torr, and the alurninium source 7 was energised to deposit a 150 A thick layer of aluminium on the mild steel plates in 2 minutes. Aluminlum deposition was termi-nated, and 500 ~ of magnesium was deposited over the alu-minium layer. 200 ~ of aluminium was then deposited over the magnesium layer. A first set of the coated plates were oxidised by leaving them in air at atmospheric pressure at 20c for 60 hours. A second set of the coated plates were oxidised by heating in air at a-tmospheric pressure at 100C
for 1 hour. Both sets of plates were activated by heating for 4 hours in a partial pressure of 4 x 10 5 Torr oxygen at 270c. The secondary emission coefficient (S ) at 500 eV
of the first set of plates was 5. 85 and was 6.15 for the second set of plates.
EXAMPLE 3.
Mild steel plates which had been plated with l/um nickel were placed in the apparatus described with referenca to Figure 1. Pressure in the apparatus was reduced to 2 x 10 Torr, and the aluminium source 7 was energised to 25~ deposit a 300 ~ Lhick layer of aluminium on the mild steel ; plates in 4 minutes. Aluminium deposition was terminated, and 800 ~ of magnesium was deposited over the aluminium layer. 300 ~ of aluminium was then deposited over the mag-; ~ nesium layer. A set o~ coated plates was oxidised by lea-ving them in air at atmospheric pressure at 1 50C for 90 minutes and the acitivated in oxygen a-t atmospheric pressure. One plate activated for 3 hours at 350C had secondary emis~ion coefficient (~ ) of 6.7 at 600 eV. An-other plate activated for 3 hours at 4000C had a ~ of 35 8. 5 at 700 eV. A plate activated 4.5 hours at 350c R S of 7.9 at 600 eV. A plate activated 6.5 hours at 350C a ~ of 8.3 at 600 eV (the voltages being the primary electron energies at which these ~ values were obtained).
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PHB 326L~9 8 4.3.80 Channel electron multiplier.
Figure 2 shows par-t of a channe:L electron multi-plier 13 built up from dynodes 1L~, 15, 16 and 17. Each of -these dynodes comprises a nickel-plated perforated steel plate, the perforations constituting channels 18 each bearing a secondary emissive coating 22 formed by a method according to the present invention. The channels 18 of the dynodes 14, 15, 16 and 17 are aligned with each other and converge in the directions of electron multiplication.
The dynodes 14 to 17 are separated by spherical separating elements 19 in the form of ballotini which are bonded by glass enamel 20 to adjacent dynodes. By way of illustra-tion the density of the elements 19 at the imperforate edges of the dynodes 1L~ to 17 is greater than in the centre thereof. Although the elements 19 are shown positioned between each channel opening of a dynode, theycould be spaced apart by integral multiples of the distance be-tween the centres of adjacent channels 18 of a dynode. Each chan-nel 15 bears a secondary emissive coating 22 formed by a 20 method according to the invention.
As thé illustrated separating elements 19 are electrically insulating, it is necessary that each dynode be biassed separately by a power supply 21. Figure 3 shows an alternative embodiment of a channel plate structure 13 to that shown in Figure 1, Dynodes 23 to 26 each comprise two, juxtaposed, mating perforated metal plates 28, 29.
Each of the channels 18 in the plate 29 and the top sur-face of each of the dynodes 23 to 26 bear a secondary emissive coating 22 formed by a method according to the in-30 vention. A single perforated metal plate 27 is disposedabove the dynode 23. The separating elernents 19 comprise ballotini arranged at suitable intervals between the chan~
nels. Once again taps of the power supply 21 are connected to respective dynodes.
Channel plate cathode-ray tube.
Eigure 4 diagrammatically illustrates a channel plate cathode-ray tube 30 comprising a metal, ~or example mild s-teel, cone 31 having a substantially flat plate glass , ~ ~63 1~) PHB 32649 9 4.3.80 screen 32 closing the open end of the cone 31. A channel elec-tron multiplier 13 as described with reference to Figure 2 is disposed at a small distance, for example 10 mrn, frorn -the screen 32. An electron gun 33 is disposed adja-cent the closed end of the cone 31 and a deflection coilassembly 34 is disposed adjacent to, but spaced from, the electron gun 33.
In operation a low energy electron beam 35 from the electron gun 33 is deflected in raster fashion across the input side of the channel electron multiplier struc-ture 13 by the coil assembly 34. The beam undergoes elec-tron multiplication in the channel electron multiplier 13 and the outpu-t electrons are applied substantially nor-mally to the screen 32.
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Claims

PHB. 32,649 10 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PRO-PERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of forming a secondary emissive coating on a dynode, characterized in that the method comprises the steps of vapour depositing a composite coating at least 200 .ANG. thick consisting of magnesium and aluminium onto the dynode, vapour depositing from 50 to 500 .ANG. of aluminium over the composite coating, oxidising the exposed aluminium layer, and activating the coated dynode by heating it in an oxygen atmosphere at a pressure of at least 5 x 10-6 Torr at a temperature between 270 and 400°C, wherein the secondary emissive coating contains from 1.5 to 90% by weight of magnesium.

2. A method as claimed in Claim 1, characterized in that the coated dynode is activated by heating it in an oxygen atmosphere of from 5 x 10-6 to 4 x 10-4 Torr.

3. A method as claimed in Claim 1 or 2, characterized in that the composite coating consists of a magnesium layer disposed on a subjacent aluminium layer which abuts the dynode.

4. A method as claimed in Claim 1, characterized in that the composite coating is formed by vapour depositing a layer of aluminium onto the dynode, and then simultaneously depositing magnesium and aluminium onto this aluminium layer.

5. A method as claimed in Claim 4, characterized in that the dynode consists at least substantially of mild steel, a 100 to 1000 .ANG. thick aluminium layer is vapour deposited onto the dynode, and a coating from 1000 to 2000 .ANG. thick of magnesium and aluminium is vapour deposited onto this aluminium layer.

6. A dynode bearing a-secondary emissive coating formed by a method as claimed in Claim 1.

7. An electron multiplier comprising dynodes as claimed in Claim 6.

8. A channel electron multiplier comprising a stack PHB. 32,649 11 of dynodes bearing secondary emissive coatings as claimed in Claim 6 characterized in that the dynodes are separated from each other by separating means disposed between each pair of adjacent dynodes, wherein each dynode substrate comprises a perforate electrically conductive sheet having a regular array of apertures, wherein the apertures of the respective dynodes are aligned so as to form the channels, wherein the separating means do not obstruct the channels, wherein the separating means are electrically insulating or have a higher electrical resistivity than that of the dynodes, and wherein the secondary emissive coatings extend at least over the walls of the apertures in the electrically conductive sheet.

9. A channel electron multiplier comprising a stack of dynodes bearing secondary emissive coatings as claimed in Claim 6 characterized in that the dynodes are separated from each other by separating means disposed between each pair of adjacent dynodes, including dynodes comprising two perforate electrically conductive mating sheets having a regular array of apertures, which two mating sheets are in electrical contact with each other, wherein the apertures of the respective dynodes are aligned so as to form the channels, wherein the separating means do not obstruct the channels, wherein the separating means are electrically insulating or have a higher electrical resistivity than that of the dynodes, and wherein the secondary emissive coatings extend at least over the walls of the apertures in the electrically conductive mating sheets.

10. A method of forming a secondary emissive coating on a dynode, said method comprising the steps of:
vapour depositing a composite coating at least 200°A thick consisting of magnesium and aluminum onto the dynode;
vapour depositing from 50 to 500°A of aluminum over the composite coating;
oxidizing the exposed aluminum layer; and activating the coated dynode by heating it in an PHB. 32,649 12 oxygen atmosphere at a pressure of at least 5 x 10-6 Torr, at a temperature between 270°C and 400°C wherein at least part of the magnesium diffuses through the oxidised aluminum coating and becomes oxidised.

11. A method as claimed in Claim 10, characterized in that during the activation step, the oxygen atmosphere has a pressure of from 5 x 10-6 to 4 x 10-4 Torr.

12. A method as claimed in Claim 10, characterized in that the composite coating consists of a magnesium layer disposed on the subjacent aluminium layer which abuts the dynode.